My very first calculator app ;)
eval('2+2');

@Nanos
Errors are any where my friend:)

Eyyy, that's pretty cool.
Check this one out I made for another comment saying a similar thing.

# Lexical Analysis,
def lex(code):
code = code.replace('(', ' ( ')
code = code.replace(')', ' ) ')
return [x for x in code.split(' ') if x]

# Parsing stage,
def parse(tokens):
tree = []
stack = []
work = tree
for token in tokens:
if token == '(':
work += [[]]
stack += [work]
work = work[-1]
elif token == ')':
work = stack.pop()
else:
work += [token]
return tree

def evaluate(ast):
for token in ast:
if type(token) is list:
return evaluate(token)
elif token.isdigit():
return int(token)
elif token == '+':
return sum(evaluate([x]) for x in ast[1:])

code = '(+ 1 (+ 1 1))'
print(evaluate(parse(lex(code))))

Now it only does addition, but wouldn't be hard to add more.